Your slot 1 is most likely dirty. If you can, get compressed air and blow it into the slot 1. It would be very likely if you don't have a game in the DS slot often, or if you place games in there that have been sitting out in the open for any length of time.
You can also force in a hard breath from your mouth. Don't be afraid to blow a dry breath onto the contacts of the games either, as those can also get kind of dirty. Make sure the breath is dry though, as you don't want to blow spit all over your games, or into your DS game slot.
Never breathe on contacts, it will make them moist and thus shorting can occur. Also, compressed air should always be blown OUT of the thing you want to clean, if you blow it inwards, it can worsen your problems.
QUOTE(bahamut920 @ Jun 17 2011, 07:12 AM) Another way to clean is to use a cotton swab and a very small amount of rubbing alcohol.